Both synthetic and biopolymers are made of repetitive monomeric units. The term primary structure is used to describe the chemical composition and the sequence of the repeated units. Many synthetic polymers prepared using petroleum based monomers have a simple, non-varied structure and are typically random copolymers where the repeat unit sequence is statistically controlled. In contrast, many biopolymers can fold into functionally compact shapes through crosslinking (via hydrogen bonding, hydrophobic associations, multivalent ion coordination, and the like). This changes not only their shape, but their chemical properties. In addition, biopolymers often have complex pendant moieties that display highly specific functionalities. The mono-dispersity and specific structure available in biopolymers provide distinct advantages over the poly-dispersity and random structure encountered in many synthetic polymers.
Rhizobium tropici ATCC® 49672 is a catalogued symbiotic nodulator of leguminous plants. Martinez-Romero et al., Rhizobium tropici, a Novel Species Nodulating Phaseolus vulgaris L. Beans and Leucaena sp. Trees, Int. J. Syst. Bacteriol. 41, 417-426, 1991. Rhizobium tropici is also known for its production of a gel-like, extracellular polymeric substance (EPS). Gil-Serrano, A. et al., Structure of the Extracellular Polysaccharide Secreted by Rhizobium leguminosarum var. phaseoli CIAT 899, Carbohydr. Res. 204, 103-107, 1990. Most of the Rhizobium-produced EPS are polysaccharides containing glucuronic acid. Dudman, W. F. et al., The Structure of the Acidic Polysaccharide Secreted by Rhizobium phaseoli Strain 127 K36, Carbohydr. Res. 117, 141-156, 1983a; Dudman, W. F., et al., The Structure of the Acidic Polysaccharide Secreted by Rhizobium phaseoli Strain 127 K87, Carbohydr. Res. 117, 169-183, 1983b; Franzén, L. E. et al., The Structure of the Acidic Polysaccharide Secreted by Rhizobium phaseoli Strain 127 K44, Carbohydr. Res. 117, 157-167, 1983. Some exceptions to this structure have been reported. Amemura, A. and T. Harada, Structural Studies on Extracellular Acidic Polysaccharides Secreted by Three Non-Nodulating Rhizobia, Carbohydr. Res. 112, 85-93, 1983; Gil-Serrano et al. (1990). Studies of the structure of these polymers have been reviewed by Laspidou and Rittmann. Laspidou, C. S. and B. E. Rittmann, A Unified Theory for Extracellular Polymeric Substances, Soluble Microbial Products, and Active and Inert Biomass, Wat. Res. 36, 2711-2720, 2002. The functions of the EPS include surface adhesion, self-adhesion of cells into biofilms, formation of protective barriers, water retention around roots, and nutrient accumulation. Laspidou and Rittmann (2002).
Refer to FIG. 5. Protonation of the hydrolytic acid functional groups 502 allows the reaction between these groups and amine functional groups 501 within the polymer, or adjacent polymers. The carboxyl group 502A reacts with the biopolymer amino group 501 releasing water 504 in a dehydration synthesis. The derivative is a polydentate ligand 503 suitable for metals sequestration.
EPS from Rhizobium tropici has unique adhesive and protective biofilm formation qualities. Given that production and transportability issues are addressed, the adhesion and water retention characteristics of ex situ “grown” EPS may be useful for dust and erosion control in situations where traditional techniques are not viable. Commercially, there are numerous products available that are employed for both dust control and soil strengthening. Nontraditional soil strengthening amendments have been investigated for many years and include ionic, enzymatic, lignosulfonate, salt, polymer and tree resin stabilizers and petroleum resins. Tingle, J. S. et al., Constitutive Analyses of Nontraditional Stabilization Additives, ERDC TR-04-5, U.S. Army Corps of Engineers, Engineer Research and Development Center, Vicksburg, Mass., 2004. These non-traditional amendments act by coating the soil particles and forming strong physical bonds with the soil. Newman, K. et al., Stabilization of Silty Sand Using Polymer Emulsions, IJP 4, 1-12, 2005. Unlike EPS biopolymers, these static molecules do not have the capacity for secondary reactions, such as crosslinking or ion exchange, which may be a key factor in strengthening the bonds between the biopolymer and the soil and have been found to be less effective when compared to petrochemical soil stabilizers. Synthetic, petroleum-based soil additives, packaged as emulsions, are gaining popularity due to their ease of handling and lower safety and environmental concerns compared to traditional soil stabilization agents such as asphalt, Portland cement, and lime. The majority of soil-stabilizing emulsions are copolymers of ethylene or vinyl acetate or are acrylic copolymers. National Resource Conservation Service, Conservation Practice Standard Anionic Polyacrylamide (PAM) Erosion Control, Code 450. These petroleum-based additives produce amended soils with improved engineering properties. Soil-stabilizing polymers, when mixed with soils, may exhibit strengths similar to that of Portland cement but impart more flexibility to the soil, i.e. increase toughness. This translates into increased resistance to cracking due to a higher ultimate failure strain before yield. Newman et al. (2005).
EPS are being investigated for use in a wide range of commercial, medical, and industrial applications. Specific applications include adsorption of heavy metals from wastewater and natural water (Comte et al., Biosorption Properties of Extracellular Polymeric Substances (EPS) Towards Cd, Cu, and Pb for Different pH Values, Jour. of Haz, Matls. 151, 185-193, 2008; Noghabi et al., The Production of a Cold-Induced Extracellular Biopolymer by Pseudomonas Fluroscens BM07 under Various Growth Conditions and its Role in Heavy Metals Absorption, Process Biochem. 42, 847-855, 2007), bioremediation of polycylic aromatic hydrocarbons in oil-contaminated beach sand (Xu et al., Use of Slow Release Fertilizer and Biopolymers for Stimulating Hydrocarbon Biodegradation in Oil-Contaminated Beach Sediments, Marine Pollution Bull. 51, 1101-1110, 2005), and treatment of activated sludge (Sheng et al., Characterization of Adsorption Properties of Extracellular Polymeric Substances (EPS) Extracted from Sludge, Colloids and Surfaces B: Biointerfaces 62, 83-90, 2008), Yu et al., Extracellular Proteins, Polysaccharides and Enzymes Impact on Sludge Aerobic Digestion after Ultrasonic Pretreatment, Water Rsch. Vol. 42 (8-9), 1924-1934, 2008).
Current materials utilized for soil stabilization and dust control are synthetic petroleum-based materials. Many of these synthetic materials are non-biodegradable and persist long after their useful life as amendments. Thus, for one application, what is needed is an environmentally benign and biodegradable replacement for petroleum-based amendments that provides increased soil strength when mixed with soil for load-bearing applications such as walkways, paths, roads, airfields and the like. For yet another application what is needed is a similar benign and biodegradable amendment to limit dust formation by agglomerating soil particles together, preventing fine particle formation which may become airborne. For yet another application what is needed is a similar benign and biodegradable amendment that may be employed for contaminant remediation. Select embodiments of the present invention address these needs.